Incinerator kilns, and especially those of the rotary type, have an outside metallic shell, usually steel, the inside of which is fully covered by a thick ceramic or refractory, usually in the form of fire bricks having a total thickness exceeding in many cases 25 cm. These kilns usually operate at a exit gas or off-gas temperature in the range of about 1,600.degree. to 2,400.degree. F. The ceramic or refractory walls, however, are very vulnerable to erosion and corrosion, due to the hostile conditions created by the nature of incinerated materials and high temperatures, especially, if alkali metals are present.
if the viscosity of slag in the kiln is adequately high, it may form a rather thick viscous coating on the refractory and thus protect it from the hostile environment. However, when the viscosity of the slag is very low, the slag contributes to the erosion and corrosion of the ceramic, both chemically because it serves as a solvent and mechanically, as it allows foreign big pieces of abrasive material to act against the ceramic walls. If the slag is viscous to the point of becoming substantially solid, or if it has never been formed as a liquid, it becomes ineffective in promoting combustion of organic matter, and also in capturing toxic heavy metals. Therefore, it is imperative that the viscosity of the slag is very carefully monitored and controlled within a range of values. Thus, one of the objects of this invention is to control the viscosity of the slag in incinerating kilns.
U.S. Pat. No. 5,301,621 (Vassiliou et al.), which is incorporated herein by reference, describes methods and devices for slag viscosity detection through image analysis of dripping slag within rotary incineration kilns.
U.S. Pat. No. 5,228,398 (Byerly et al.), which is also incorporated herein by reference, describes methods and devices for controlling rotary incineration kilns by determining the position of the kiln outlet at which the slag is exiting.
Our U.S. application Ser. No. 08/106,536, which is also incorporated herein by reference, describes preventive slag-viscosity control by detection of alkali metals in the off-gases.
The aforementioned references, however, do not address the problem of uncontrollably raised temperatures in the kiln, which regardless of the presence of alkalis or other factors, are detrimental to the refractory. Not only the slag viscosity decreases drastically at higher temperatures, but also the reactivity of the slag increases accordingly, accompanied by substantial decrease in the resistance of the refractory to chemical and mechanical attack.
The problem is even more profound when the feeding of waste to the kiln is not controllably continuous but it is incremental, which is very often necessary, especially with solid or semisolid waste. "Controllably continuous" means that the waste is initially shredded, blended with other types of waste or additives, and then fed to the kiln by means of screw-type or other type feeder, well known to the art. Incremental feeding is conducted by dropping incremental quantities of waste to the kiln, such as drums and the like, for example, containing waste. Incremental feeding is many times necessary for a number of reasons. One important reason is that the capital equipment cost of shredders and feeders required for continuous feeding of solids or mixtures of solids and liquids is very high, and many environmental facilities do not have such equipment. Another reason is often that it is hazardous and/or highly undesirable to shred certain types of already packaged solid or semisolid waste. Thus, although liquid waste may be introduced to the incinerator in most cases easily by means of appropriate lances, and the like, feeding the incinerator in incremental quantities presents many problems. Control of the kiln temperature in case of liquid waste or even continuous feeding of shredded solid waste is rather easy, since monitoring the temperature at different points of the kiln gives enough information to the operator or to an automatic control device to manipulate the waste feeding rate, the supplementary fuel and the oxidant feeding in order to closely control the temperature, and in general the burning rate. Thus, the burning rate the case of continuous feeding may be controlled just by adjustment of the flow rates of the different aforementioned feeds.
However, after a drum has been dropped into the kiln, it is too late to take any practical measures to control effectively the temperature, in case that the drum happened to contain a large amount of vigorously burning waste. Despite of even completely turning off any supplementary fuel burners and/or liquid waste, uncontrolled burning of the "hot" drum may cause serious problems, such as extreme temperature hikes, unacceptable slag-viscosity decrease, refractory damage, and the like. In some occasions, the release of uncontrolled heat may even reach the proportions of explosion.
Thus, a way to prevent this from occurring is needed.
U.S. Pat. No. 4,724,775 (May) discloses a method and apparatus to control the rate of heat release (ROHR) from a reaction zone confined within a chamber, air and fuel having been introduced into said reaction zone to achieve combustion, plural sensors being utilized, one sensor adapted to sense ROHR, another to generate a target value for a desired ROHR, within a comparator, still another sensor to determine heat generation and transmit it to the comparator, a control means for controlling ROHR of heat generation and connecting said comparator with said control means to achieve coincidence between a target value and said sensed value.
U.S. Pat. No. 4,038,032 (Brewer et al) discloses a control system for the incineration of pollutants in waste gases which will conserve fuel consumption and which embodies feedback signals from temperature sensing means and/or gas analyzing means in connection with the combustion gases to detect an undesired temperature deviation from a control temperature or, alternatively, detect excessive unburned pollutants and utilize optimizing-controller means to receive the resulting output signals and, in turn, provide internal control changes to effect a change in the temperature control level and the incremental stepping down or stepping back of a set point of the control of fuel flow to the heat supplying burner of the incineration zone.
U.S. Pat. No. 4,794,870 (Visvesvaraya) discloses a system for modulating the firing temperature in a rotary kiln. The method comprises determining the absolute content of one or more inorganic constituents present in the coal feed for determining the total ash content in the coal. In the event that the temperature in the kiln is different from the required temperature, then a sweetener fuel is fed to the burner with or without a change in the flow of coal. Alternatively, only the amount of flow of coal to the burner is reduced or increased.
According to U.S. Pat. No. 4,739,714 (LaSpisa et al.), the flow rate of multiple fuel streams supplied to an incinerator are controlled so as to provide maximum temperature and heat release conditions in the incinerator that will allow complete combustion of hazardous waste fuel. In addition, a minimum temperature for the incinerator is maintained by manipulating the flow rate of an auxiliary fuel such as natural gas, that is also supplied to the incinerator. In use, a control signal for manipulating a waste fuel flow is selected as the lowest signal of signals representative of a maximum temperature for the incinerator, a maximum heat release rate for a particular waste fuel, a maximum pressure for the incinerator, and the combustion air available.
U.S. Pat. No. 3,605,655 (Warsshawsky et al) discloses a control system for a fluidized bed reactor which is used for treating material. Although the control system is primarily designed for use with a reactor which is used for incinerating combustible wastes such as sewage sludge, oily wastes such as oily wastes from an oil refining operation or other combustible wastes, it may also be used with a reactor which is being used for other high temperature operations such as calcining operations. Fuel and quench water are supplied to the reactor to maintain a desired temperature within the fluidized bed. The temperature of the fluidized bed is measured and compared to a set temperature. The difference between the actual temperature of the fluidized bed and the desired temperature controls the supply of either fuel or quench water to the reactor to maintain the actual temperature of the bed equal to the desired temperature. A controller responsive to the difference between the desired temperature of the freeboard area and the actual temperature controls the volume of wastes supplied to the reactor.
The aforementioned references attempt to control the temperature in miscellaneous operations, which, however, do not confront the problem of incremental waste feeding or other similar problems, as will be discussed later.
In contrast to the art cited above, the instant invention deals directly and resolves the problems arising from temperature effects, especially in feeding waste in increments.